Configuring Earth Constants

This procedure is used to configure the earth constants for controlling the corrections applied between ground distances and plane distances. You may obtain additional help by clicking the underlined topics. If you do select an underlined topic, you may return to this procedure by clicking the Back key.

  1. When creating a new database via the Utilities/File/New command select the Earth Constants button to display the Configure/Earth Constants dialog box.
    Alternatively the dialog box may be run at any time via the Utilities/Configure/Earth Constants command.
    NOTE: be very wary of changing the settings in this dialog box after the database has been created and used, as to do so changes the mathematical relationship between plane and ground distances.
  2. Select the refraction method and enter a value for Refraction Constant (K).
    This value will be used if curvature and refraction correction is applied when the field file is reduced.
  3. If the database is a projection, enter values for:
    • Average Elevation
    • Geoidal Separation
    These values will be used if mean sea level correction and geoidal separation correction are applied when the field file is reduced. They are also needed by the system to allow display and entry of ground distances. Average elevation is needed to allow display of ground distances and calculation of sea level correction, when a point does not have an elevation.
    If the database is a plane and Combined Scale Factor is not to be used, ensure that the "Use Combined Scale factor always" check box is not selected, and enter values for:
    • Earth Radius (R)
    • Average Elevation (H)
    • Geoidal Separation
    • Point Scale Factor
    Earth Radius, Average Elevation and Geoidal Separation values will be used if mean sea level correction and geoidal separation corrections are applied when the field file is reduced. They are also needed by the system to allow display and entry of ground distances. Average elevation is needed to allow display of ground distances and calculation of sea level correction, when a point does not have an elevation.
    Point Scale Factor is also needed to allow display of ground distances.
    Note: To define the data base plane at the geoid, set geoidal separation to zero, and Point scale factor to 1.
    To define the database plane at the average elevation, set geoidal separation to zero, and point scale factor to (R+H)/R
    If the database is a plane and Combined scale factor is to be used, ensure that the "Use Combined Scale factor always" check box is selected and enter a value for Combined Scale Factor.
    The combined scale factor will then be the only difference between plane and ground distances. Sea level correction, geoidal separation and point scale factor have no relevance, and the point elevations are ignored when displaying ground distances.
    Note: To define the database plane at the ground, set the combined scale factor to 1.
  4. Select the OK button to accept the changes and close the dialog box.
    Refer to the Configure/Earth Constants dialog box help topic for more details.